Summary
Highlights
Drug metabolism is the chemical conversion of drugs into water-soluble compounds for excretion, mainly occurring in the liver. Drugs are typically lipophilic and can cross cell membranes, but must become hydrophilic for kidney excretion.
Drugs are administered orally or parenterally. Parenteral routes bypass the digestive tract, leading to quick systemic circulation. Oral drugs pass through the GI tract to the liver via the hepatic portal vein, where they undergo first-pass metabolism before entering general circulation.
In the liver, hepatic microsomal enzymes like p450 in the smooth endoplasmic reticulum convert lipophilic drugs into hydrophilic compounds. Repeated drug use can increase these enzymes (enzyme induction), while some drugs compete for binding, decreasing metabolism (enzyme inhibition).
A drug's duration and intensity are determined by its metabolic rate. Increased metabolism decreases duration and intensity, while decreased metabolism increases them. Factors affecting metabolic rate include the first pass effect, liver disease, age, blood flow to the liver, enzyme variations, diet, and drug dosage/patient weight.